As generally accepted, the most reliable method to measure the blood pressure is by indirect auscultation. The technique consists of using an air pressure cuff as an artery occlusion device and a stethoscope for detecting the Korotkoff sounds. The inflatable cuff is applied surrounding the upper portion of the patient's arm. A hand-held air pump is used to quickly inflate the cuff to a pressure above the presumed systolic pressure by about 20-30 mmHg. Thereafter, the pressure in the cuff is gradually decreased at a rate of 2-3 mmHg/seconds by means of a bleeding valve. A point is reached at which the occluded artery begins to open briefly during each cardiac systole. At this point, the air pressure in the cuff is considered equal to the blood pressure in the brachial artery and is accepted in the medical art as systolic blood pressure.
As the pressure in the cuff continues to drop, a point is reached at which the brachial artery becomes fully open during the entire cardiac cycle. The heart is at rest at this time. This pressure is understood as diastolic blood pressure.
As mentioned above, the operator uses a stethoscope to detect the Korotkoff sounds. Korotkoff sounds are caused by the pulsating blood flow during the cardiac cycle. These sounds are generated according to a characteristic sequence well established since Korotkoff proposed his auscultation method: sound suddenly appears, changes in character, and gradually disappears. Systolic pressure is read when the first of two consecutive sounds appears during the deflation procedure. Diastolic blood pressure corresponds to the "muffling" of sound (first diastolic) and the total disappearance of sound (second diastolic).
The listening device--the cup of the stethoscope is applied over the artery, downstream to the inflated cuff to pick up the sounds generated by the pulsating blood flow in the compressed artery. However, Korotkoff sounds are low in amplitude and frequency with the band-width extending from 25 to 200 Hz. It is almost at the limits of the capability of the human ear. It is hard for the human ear to extract the true Korotkoff sounds from the background noises. Medical personnel making such measurements are required to make rather difficult determinations regarding the presence or absence of Korotkoff sounds in the context of ambiguous signals generated by artifacts. In other words, there are risks of misinterpretation and inaccuracy in the absence of experience and good training on the part of the operator.
Unfortunately, the majority of hypertensive patients have no formal training on how to use the instrument and above all, cannot afford to see the doctor on a regular basis. Therefore, they choose to check their blood pressure at home, using the conventional cuff and the very familiar stethoscope despite the lack of training in that operation.
To date, various attempts have been made to help hypertensive patients to check their own blood pressure and to eliminate the aforementioned deficiencies by mechanizing the technique and apparatus. The easiest instruments to use, though very costly, are electronic with digital displays, and LED's to provide readout of systolic and diastolic levels. Nevertheless, these electronic devices are overly sensitive to artifacts and noises and appears in many instances less accurate than the manual procedures. Consequently, automatic devices have experienced only limited acceptance by the medical profession. Doctors and nurses in the hospital and hypertensive patients at large remain fidel to the conventional method, using the blood pressure cuff and the stethoscope.
The stethoscope is primarily an instrument used for auscultation of the heart and the lung. The diaphragm housed in the cup of the stethoscope. scope is a flat surface of 3 cm. in diameter. The operator presses it very slightly to the chest wall which is another flat surface, so as to have an air seal, within the full rim, in order for the cup not to pick up noise from the environment. Now that the instrument is used to check the Korotkoff sound during the blood pressure measurement, the difficulty arises when the diaphragm is applied to the arm which is a curved surface, such air seal is obtained only when the operator presses firmly. This in turn, creates some kind of additional pressure to the brachial artery which alters the normal sequence of production of Korotkoff sounds, adding more inaccuracy in the reading.
An individual taking his own blood pressure, usually will have his arm completely in a resting position. The other hand is used to operate the air pump to inflate or deflate the cuff. He still needs an assistant to hold the cup of the stethoscope on the brachial artery. Obviously, self-taking blood pressure measurement is an impossibility with the conventional apparatus if an assistant is not available.
To overcome the above noted difficulties, the operator usually tries to insert the cup of the stethoscope between the cuff and his arm to have a free hand. By doing so, he reduces the cuff width. The length of the arterial segment which is compressed by the cuff is an important factor that influences the accuracy of blood pressure measurement. This segment is determined by the width of the occluding cuffs. A standard cuff width should be 40% of the circumference of the arm, grossly 13 cm. for the adult. As the stethoscope cup if 4 cm. (diaphragm and peripheral rim), the insertion of that cup reduces the width of the cuff that becomes 13-4=9 cm; causing additional source of error in the measurement of blood pressure.
During the deflation of the cuff, the operator uses his thumb and index finger to loosen the valve. As air is escaping, the pressure in the cuff should drop slowly, no more than five points per second: too rapid deflation causes error in reading and too low deflation results in venous congestion and causes false readings. A non-skilled person needs a controlled valve that allows air to escape at a constant rate.